The present invention is related to a zoom lens and particularly to a zooming system to perform the function of a focusing lens after a variator lens group.
There have been a number of lens system of the zooming type proposed, for example, a zoom lens system suited for wide angle zooming or a zoom lens system with the nearest shooting distance which is short and with a small outer diameter of lens. Such systems all make correction of a focusing plane in a mechanical manner. A prior art lens system is composed of a focusing lens group, a variator lens group, a compensator lens group and an imaging lens group.
Of these basic four groups, the focusing lens group operates so that each object a different respective distance will not cause the shifting of image, and the compensator lens group prevents the shifting of image during zooming with respect to one object, and, in general, stabilization of focusing is done by so moving the compensator lens group along a curved cam.
Also in a zoom lens system, with an exception of a lens system with a low magnification, a focusing group ordinarily has a positive refractive power for a requirement for compactness of a device, while a variator lens group which has a function of magnification variation has a negative refractive power.
Therefore, as has been known as a general characteristics of a zoom lens, the shorter the object distance, the more the focusing lens has to be moved out and, as a result, the outside diameter of the lens required will sharply increase. Accordingly, it has been difficult to make the object distance which allows photographing as short as possible, that is, to make the nearest distance as short as possible, and at the same time, to make the lens part small, as desired by a user of camera.
Contrary to this, when a focusing function is provided for the part after the variator lens groups, the outer diameter of lens will not sharply increase even for the nearest distance. The reason why such rear focus system as mentioned above is not employed is that, if an object position is different for one zooming position, the amount of shifting of the focusing lens group will be different.
Furthermore, when the zooming position changes as zooming is done, the amount of shifting of a focusing group will become different with respect to such object distance as being same as that in the zooming position. Therefore in practical photographing, since there are a number of such focal distances, that the object distance can be selected from the nearest distance to the infinite and the zooming position can be selected from wide angle to telescopic, it is deemed impossible practically to change the amount of shifting of the focusing group at the same time zooming is done, further and to change the amount of shifting of the focusing group depending on the object distance mechanically. While there is a lens system where a focal point is adjusted by moving the lenses behind a variator lens, as a zoom lens by which close photographing is possible, such system inevitably will have the shifting of the focal point taking place during zooming, and except as used for special photographing purpose, it cannot be used as an ordinary zoom lens. Thus, the part before the variator lens has generally had of a zoom lens the focusing function, or a zooming part and a focusing group has been used in common taking advantages of the depth of field of a lens system for using the same for a zoom lens with low magnification and dark F-number, as a zone focusing system.